Drill with a hard material drilling head

ABSTRACT

A one-piece drilling head ( 4 ) which is to be secured, in a material-locking manner, at an end of a helical shaft of a drill, is formed of hard material and includes four radially extending, in a cross-shaped manner, legs ( 5 ) forming, respectively, two diametrically opposite main cutting edges ( 6 ) and two auxiliary cutting edges ( 7 ), with the main cutting edges ( 6 ) and the auxiliary cutting edges ( 7 ) extending proceeding from a drilling head drilling head tip ( 8 ), monotonously and strictly axially up to a radially outer edge, and with an envelope surface ( 9 ′) of the auxiliary cutting edges ( 7 ) lying completely within an envelope surface ( 9 ) of the main cutting edges ( 6 ) and contacts the same only at the drilling head tip ( 8 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a boring tool with a hard material drilling head and, in particular, to a rotary-percussion drill for stone and reinforced concrete and to be used with a high-power hand-held power tool. More specifically, the present invention relates to the hard material drilling head for the rotary-percussion drill.

2. Description of the Prior Art

Usually, a drill which is mentioned above, has a helical shaft extending axially along a rotational axis of the drill and provided on its power tool-side end with a shank for insertion in a chuck of the power tool and at its opposite workpiece-side end with a hard material head brazed to the shaft. The drilling head has four radially extending, in a cross-shaped manner, legs forming, respectively, two diametrically opposite main cutting edges and two auxiliary cutting edges, and a drilling head tip.

International Publication WO 2005/061838 discloses a rotary-percussion drill with a one-piece head formed of hard material and having four cutting edges which are arranged in a cross-shaped manner and are convexly curved in the rotational direction, and a drilling head tip.

European Publication EP 1 604 762 discloses a drill for stone with a one-piece hard material drilling head butt-brazed to the shaft and having four legs arranged in a cross-shaped manner and forming two diametrically opposite straight main cutting edges and two diametrically opposite auxiliary cutting edges concavely curved in the rotational direction. The auxiliary cutting edges have a ramp surface tangentially inclined in the rotational direction (for axially lifting the drill at the power-tool side upon encountering a reinforcing iron) and extend exclusively in the radially outer region. Two diametrically opposite drilling head cutting edges, which extend axially linear to respective drill tips formed by the main cutting edges, are provided only in the radially inner drilling head region of the drilling head. The radially inner envelope surface of the drilling head region has a more acute ridge angle than the radially outer envelope surface of the legs.

An object of the invention is to increase the drilling capacity of the drill of the type described immediately above.

Another object of the invention is to increase robustness of the drill upon encounter with reinforcing iron.

SUMMARY OF THE INVENTION

These and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a one-piece, hard material drilling head having four radially extending, in a cross-shaped manner, legs forming, respectively, two diametrically opposite main cutting edges and two auxiliary cutting edges, and a tip. The main cutting edges and the auxilially cutting edges extend, proceeding from the drilling head tip, monotonously and strictly axially (i.e., only regarding the axial components) up to a radially outer rim of the drilling head in envelope surface of the auxiliary cutting edges lies completely within an envelope surface of the main cutting edges and contacts the envelope surface of the main cutting edges only at the drilling head tip.

With the envelope surface of the auxiliary cutting edges lying within the envelope surface of the main cutting edges, the radial standardized ridge angle of the auxiliary cutting edges is smaller than that of the main cutting edges. Thereby, with the outer radial region of the drilling head substantially defining the drilling capacity, the press-on force is concentrated on the main cutting edges, whereas the auxiliary cutting edges serve for protecting the drilling head upon encounter with reinforcing iron. All of the four cutting edges provide for good centering and guiding characteristics in the inner central region.

Advantageously, the angle difference of radial standardized ridge angles of the envelope surface of the main cutting edges and the envelope surface of the auxiliary cutting edges lies in a range between 2° and 10°. This angle difference permits to achieve an optimal drilling capacity and robustness.

Advantageously, the main cutting edges and the auxiliary cutting edges extend axially linearly, whereby their envelope surfaces are formed as strictly conical surfaces.

Advantageously, the main cutting edges form, proceeding from the drilling head tip, a straight line in an axial projection. Thereby, the main cutting edges have a minimal length and a maximal, linear force density at a given press-on force.

Advantageously, the auxiliary cutting edges form, proceeding from the drilling head tip, in an axial projection, a convexly curved line facing a following (in the rotational direction) main cutting edge. Thereby, the drillings, which are primarily removed by the main cutting edge (following the auxilially cutting edge in the rotational direction) are better advanced in the helical shaft of the drill to which the drilling head is attached.

Advantageously, the auxiliary cutting edges, proceeding from the drilling head tip, are uniformly curved (i.e., with a constant curvature radius). Thereby, a more pronounced curvature at local points of the auxiliary cutting edges, where the drillings can accumulate, is avoided.

Advantageously, the leg of the main cutting edge forms, at the outer radial rim on its side facing in the rotational direction, a tangential ramp surface axially spaced from the main cutting edge. Thereby, a ramp surface is also formed on the main cutting edge and which prevents seizure of the outer corner of the main cutting edge in ductile reinforcing iron upon a sidewise encounter with the reinforcing iron.

The novel features of the present invention which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiment, when read with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings show:

FIG. 1 a a front view of a drilling head of a drill according to the present invention;

FIG. 1 b a side view of the drilling head shown in FIG. 1; and

FIG. 1 c a plan view of the drilling head shown in FIGS. 1-2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A drilling head 4 according to the present invention, which is shown in FIGS. 1 a, 1 b, and 1 c, is formed of a hard material and is butt-brazed at an end 3 of a helically shaped shaft 2 of a drill 1 rotatable about a rotational axis A and percussively displaceable therealong. The hard material drilling head 4 has four, radially extending in a cross-shaped manner, legs 5 with two main cutting edges 6 and two auxiliary or minor cutting edges 7. The main cutting edges 6 and the auxiliary cutting edges 7 extend axially linearly from a drilling head tip 8 up to the outer radial rim of the drilling head, with an envelope surface 9′ of the auxiliary cutting edges 7 lying completely within an envelope surface 9 of the main cutting edges 6 and contacting the envelope surface 9 of the main cutting edges 6 only at the drilling head tip 8. The angular difference of ridge angles, which are inscribed in the conical tip, between the envelope surface 9 of the main cutting edges 6 and the envelope surface 9′ of the auxiliary cutting edges 7 amounts to 130°−125°=5°. The main cutting edges 6 form, in an axial projection, a straight line extending from the drilling head tip 8. The auxiliary cutting edges 7 are uniformly convexly curved, in a drill rotational direction ω, with respect to respective following main cutting edges 6. The leg 5 of the main cutting edge 6 forms, at the outer radial rim on its side facing in the rotational direction ω, a tangential ramp surface 10 axially spaced from the main cutting edge 6.

Though the present invention was shown and described with references to the preferred embodiment, such is merely illustrative of the present invention and is not to be construed as a limitation thereof and various modifications of the present invention will be apparent to those skilled in the art. It is therefore not intended that the present invention be limited to the disclosed embodiment or details thereof, and the present invention includes all variations and/or alternative embodiments within the spirit and scope of the present invention as defined by the appended claims. 

1. A one-piece drilling head (4) for a drill and formed of hard material, comprising four radially extending, in a cross-shaped manner, legs (5) forming, respectively, two diametrically opposite main cutting edges (6) and two auxiliary cutting edges (7), and a drilling head tip (8) wherein the main cutting edges (6) and the auxilially cutting edges (7) extend, proceeding from the drilling head tip (8), monotonously and strictly axially up to a radially outer rim, and an envelope surface (9′) of the auxiliary cutting edges (7) lies completely within an envelope surface (9) of the main cutting edges (6) and contacts the envelope surface (9) of the main cutting edges (6) only at the drilling head tip (8).
 2. A drilling head according to claim 1, wherein an angular difference of radial standardized ridge angles of the envelope surface (9) of the main cutting edges (6) and the envelope cutting surface (9′) of the auxiliary cutting edges (7) lies in a range between 2° and 10°.
 3. A drilling head according to claim 1, wherein the main cutting edges (6) and the auxiliary cutting edges (7) extend axially linearly.
 4. A drilling head according to claim 1, wherein the main cutting edges (6) form, proceeding from the drilling head tip (8), a straight line in an axial projection.
 5. A drilling head according to claim 1, wherein the auxiliary cutting edges (7) form, proceeding from the drilling head tip (8), in an axial projection, a convexly curved line facing a following main cutting edge (6).
 6. A drilling head according to claim 5, wherein the auxiliary cutting edges (7) are uniformly curved.
 7. A drilling head according to claim 1, wherein a leg (5) of the main cutting edge (6) forms, at the outer radial rim on a side thereof facing in the rotational direction (ω), a tangential ramp surface (10) axially spaced from the main cutting edge (6).
 8. A drill, comprising a helically shaped shaft (2); and a one-piece drilling head (4) formed of a hard material and secured on an axial end (3) of the shaft (2) in a material-locking manner, the drilling head including four radially extending, in a cross-shaped manner, legs (5) forming, respectively, two diametrically opposite main cutting edges (6) and two auxiliary cutting edges (7), and a drilling head tip (8), wherein the main cutting edges (6) and the auxilially cutting edges (7) extend, proceeding from the drilling head tip (8), monotonously and strictly axially up to a radially outer rim, and an envelope surface (9′) of the auxiliary cutting edges (7) lies completely within an envelope surface (9) of the main cutting edges (6) and contacts the envelope surface (9) of the main cutting edges (6) only at the drilling head tip (6). 